Shortages of fossil fuel materials in the recent past has spurred much speculation regarding the feasibility of economies based on other energy sources. One such scenario is a hydrogen-fueled economy. Hydrogen has the highest energy density per unit weight of any chemical. Many projections have been made for an economy based on this element, but the technology is not yet in place to effect such a dramatic change in the world economy. Hydrogen is, however, a technically attractive source of fuel and energy storage. It is essentially non-polluting, the major byproduct of combustion being H.sub.2 O, and can be made from readily available and abundant raw materials.
While it is well known that hydrogen can be stored as a compressed gas or cryogenically as a liquid, other less energy-intensive and more convenient means are required for widespread utilization of hydrogen as a source of stored energy.
It is known that some metals and metal alloys are capable of storing hydrogen reversibly within their lattice. This characteristic may be exploited by exposing the metal or metal alloy to a large pressure of hydrogen, impregnating the metal or metal alloy with hydrogen and later recovering the stored hydrogen or subjecting the impregnated metal or alloy to a change in temperature or pressure. One example of a metal that is capable of reversible hydrogen storage is palladium which can absorb up to 0.6 hydrogen atoms for every palladium atom. For an example of reversible hydrogen storage alloys, see, R. L. Cohen and J. H. Wernick, "Hydrogen Storage Materials: Properties and Possibilities", Science, Dec. 4, 1981, Vol. 214, No. 4526, pg. 1081, which reported on the ability of alloys such as LaNi.sub.5 to absorb hydrogen in the gas phase.
This characteristic of reversible hydrogen storage for LaNi.sub.5 -type alloys was reported as also being applicable in an electrochemical environment by Bronoel et al., "A New Hydrogen Storage Electrode", International Journal of Hydrogen Energy, Vol. 1, pp. 251-254, 1976. A metal or metal alloy suitable as a hydrogen storage material may be cathodically biased relative to a suitable counter electrode and charged with hydrogen by the reduction of a proton from solution. Other metal alloy systems that have been studied included TiMn-based, FeTi-based and Mg-based alloys. Although some of these crystalline materials store appreciable quantities of hydrogen, these same crystalline materials are susceptible to phase separation, hydrogen embrittlement and surface oxidation when subjected to repeated charge/discharge cycles for hydrogen storage. Phase separation occurs in crystalline alloys that are subjected to hydrogen cycling, wherein the alloy components separate and migrate throughout the alloy. In LaNi5-type alloys, La migrates to the surface of the alloy, where it may rapidly become oxidized.
This problem was recently adddressed in Japanese Publication 58, 163,157 entitled "Metal Oxide-Hydrogen Battery". This publication describes a hydrogen storage battery having an improved LaNi5 anode that is less susceptible to oxidation. This improvement comes from the use of a porous nickel layer disposed around the LaNi5 anode to reduce oxidation.
Hydrogen embrittlement occurs in crystalline alloys as hydrogen is absorbed and desorbed. Hydrogen storage proceeds from the surface of the alloy to its interior, with hydrogen atoms breaking into the interstitial site of metal matrix atoms and thus expanding the lattice. As a result, internal stresses may produce flaws and cracks, seriously weakening and embrittling the metal or metal alloy. Surface oxidation may occur if the hydrogen storage material is exposed to oxidative conditions in the presence of an oxidant such as CO.sub.2, H.sub.2 O, KOH, air, oxygen, or an oxidizing acidic environment. Surface oxidation interferes with the penetration of hydrogen, reducing the amount of hydrogen absorbed and the rate of absorption. Additionally, these crystalline materials generally cannot withstand corrosive environments, which environments may exist when the materials are utilized in an electrochemical reaction. An analysis of the TiMn alloy system, and its attendant drawbacks, is provided in Yayama, et al., "Electrochemical Hydrogen-Storage in Ti-Mn Alloy Electrodes", Japanese Journal of Applied Physics, Vol. 22, No. 10, pp. 621-623, Oct. 1983.
Recently, amorphous metal alloy materials have been reported as having the ability to store hydrogen reversibly. Amorphous metal alloy materials have become of interest due to their unique combinations of mechanical, chemical and electrical properties. Amorphous metal materials have compositionally variable properties including high hardness and strength, flexibility, soft magnetic and ferroelectric properties, very high resistance to corrosion and water, unusual alloy compositions, and high resistance to radiation damage. The unique combinations of properties possessed by amorphous metal alloy materials may be attributed to the disordered atomic structure of amorphous materials that insures that the material is chemically homogeneous and free from the extended defects that are known to limit the performance of crystalline materials.
A general discussion of hydrogen absorption by amorphous or glassy metal alloys was provided by G. G. Libowitz and A. J. Maeland, "Interactions of Hydrogen with Metallic Glass Alloys38 Journal of the Less-Common Metals, 101, pp. 131-43, 1984.
Schroeder and Koster studied hydrogen embrittlement in Fe-Ni-B, Pd-Zr amorphous alloy ribbons, "Hydrogen Embrittlement of Metallic Glasses", Journal of Non-Crystalline Solids, 56, pp. 213-218, 1983. Whereas Fe-Ni-B alloys exhibited low hydrogen absorption and severe embrittlement, Pd-Zr and Ni-Zr alloys could absorb up to one atom of hydrogen per metal atom and still retain some ductility.
Amorphous metal alloy systems of TiCu and ZrCu were investigated and contrasted with the absorption properties of the corresponding crystalline intermetallic compounds by Maeland, et al., "Hydrides of Metallic Glass Alloys," Journal of the Less-Common Metals, 74, pp. 279-285, 1980. Amorphous metal alloy compositions, under similar conditions of temperature and pressure were capable of absorbing larger amounts of hydrogen than their crystalline counterparts. Maeland, et al. restricted their studies to the gaseous absorption of hydrogen in a hydrogen atmosphere. The amorphous compositions are not expected to suffer from phase separation or to become embrittled, due to their unique structure. However, these materials may not show substantial resistance to surface passivation by oxidation or to corrosion. Maeland, et al., by excluding oxygen in their system, and by working in a gaseous environment, have avoided addressing the effects of oxidation and harsh environments on the hydrogen storage amorphous metal alloys that were investigated.
A patent publication in the United Kingdom, GB 2 119 561 A, to Energy Conversion Devices, Inc., describes a battery utilizing a hydrogen rechargeable anode that is an amorphous metal material. This publication examined Ti-Ni and Mg-Ni compositions as hydrogen storage anodes.
U.S. Pat. No. 4,637,967, also to Energy Conversion Devices, Inc., discloses an amorphous metal electrode, composed of Ni, Ti and a third element selected from a large group of elements, usable in energy storage devices to store hydrogen.
The work recited hereinabove is an indication of the interest that lies in the field of energy storage through the use of reversible hydrogen storage materials. However, the ability to store hydrogen is not alone sufficient to yield a useful material having widespread applications. The stability of such a material is also of paramount importance. Resistance to corrosion and oxidation must exist for continuous full-cycling of these materials. It is noted that the hydrogen batteries described in Japanese Publication 58,163,157 and U.K. Publication GB 2119561 A are never fully discharged during cycling, the fully discharged materials being sensitive to oxidation, and thus, to failure.
Thus it is seen that the potential exists for significant new technological advances in the application of amorphous metal alloys to the development of hydrogen as a fuel and a source of stored energy, especially in the area of electrochemical reversible hydrogen storage. What is needed in this area are amorphous metal alloy compositions having the ability to reversibly store hydrogen in significant quantities. Such amorphous metal alloys should not suffer from phase separation or hydrogen-caused embrittlement, nor be susceptible to surface oxidation or corrosion. Further, what is needed in the field of reversible hydrogen storage for energy storage devices is an electrode, operable in an alkaline or acidic environment, that can reversibly store hydrogen and undergo deep cyclic discharges without becoming unstable or detrimentally affected by corrosion or oxidation.
It is therefore one object of the present invention to provide amorphous metal alloy compositions and electrodes made therefrom capable of reversibly storing hydrogen.
It is an additional object of the present invention to provide amorphous metal alloy compositions capable of reversibly storing hydrogen in a cyclical fashion without becoming embrittled or suffering from a phase separation.
It is another object of the present invention to provide amorphous metal alloy compositions and electrodes made therefrom capable of reversibly storing hydrogen in a cyclical manner that are not impaired by surface oxidation or corrosion.
It is yet another object of the present invention to provide a hydrogen storage electrode that can be used cyclically through deep discharge cycles, and in an energy storage device that can cyclically be charged to high energy densities and deeply discharged.
These and other objects of the present invention will become apparent to those skilled in the art from the description that follows and the appended claims.